. • . LOFT ORNL P 2239 1 S ... . . . • I . . PFEFECTE 1 : MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS - 1963 O River ornerph Conf.660577 MASTER JUN 27 1966 To be submitted for publication in the August issue oi the IEEE's Transactions on Nuclear Science, Proceedings of the International Conference on Isochronous Cyclotrone, Gatlinburg, Tennessee, . May 2-5, 1966. CFST US! HC $ 1.00 OPERATING CHARACTERISTICS OF THE ORIC BEAM EXTRACTION SYSTEM N. 50 E. D. Hudson, R. S. Lord, W. H. White, Jr. Oak Ridge National Laboratory Oak Ridge, Tennessee Introduction Operation The ORIC extraction system was described previouely in its component parts. Several modifications have been performed and experience has been gained in the operation of the system. The system shown in Figure I consists of both electrostatic and electromagnetic elements. The electrostatic channel consists of ar. aluminum deflector electrode, water cooled, and operated at 60 to 70 kV across a gap of 1 to 1.5 cm to a grapnite scptum 0.8 mm thick. Both the septum and doflector curvature can be varied oy a remote control system from the control room. The coaxial magnetic channel follows the electrostatic channel and provides a 4 kG maximum field reduction over a length of 22 cm. The third element of the extraction system, a compensated. iron channel, can provide up to 9 KG of field reduction along the path of the deflected bear... In operation, the system has provided ex- tracted beams of protons from 18 to 65 MeV, deuterons from 22 to 40 MeV, alpha particles from 44 to 80 MeV, and 'He particles from 25 to 55 MeV. Extraction efficiency is 60% for most particles but drops to ~30% for the highest and lowest energy protons. Lower energies for elm = 1/2 particles could be obtained but no demand for these energies exists. Also, no program at present requires 'He above 55 MeV, but no problem is anticipated in obtaining 100 MeV for this particle. To minimize the divergence of the extracied beam the pole tips are oriented so that the major portion of the fringe field is crossed at :ight angles to field contour lines, as shown in Figure 1. The radial divergence, as a result of this is less than +0.6° The modifications to the above system in- clude the incorporation of the curvature adjust- ments on the electrostatic channel, a remotely controlled motion adjustment for the exit of che compensated-iron channel and improvements in the water cooling lead arrangements of the co- axial channel. The first two of these allow greater flexibility in the range of particles which can be extracted. The water lead modification remedied a propensity for leaking which caused a considerable loss of operating time. The initial decision to begin extraction at a radius where v is still greater than 1.0 was made to avoid the loss of beam associated with passing through the outer resonances. It was found, while the extraction system was s'ill under construction, that the introduction of a harmonic in the magnetic field by coils near the center region produces controllable and coherent og cillations about the magnetic center. These oscillations can provide scod orbit separation at the entrance of the septum. The value of v must remain relatively constant and be slightly greater than 1.0; the amplitude of the oscillations must remain within the stability limits of the phase- space region, and the phase width of the beam must te relatively narrow to prevent loss of coherence. If liese conditions are available, controllable linear oscillations can be provided by use of the harmonic coils, and good extraction efficiency can be obtained. The effect of this oscillation is shown in Figure 2. For this case v is about 1.05 in the region of the extraction radius. The vari- ation of beam density vs icdius is indicated by the current on probe 2 where the radial width exceeds 3/8 inch. The total current on probes 1 and 2 shows small dins where the beam density is high- est. These dips result from increased scattering from the probe leading edge and provide a good indication of structure without the segmented Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation. LEGAL NOTICE RELEASED FOR ANNOUNCEMENT : IN NUCLEAR SCIENCE ABSTRACTS The report mo prepared us account of Government sponsored work. Neither we Vallad Halos, bor ibe Commission, nor any person scuing oa berall of the Commission: A. Makes say w rity or representation, 4Xpruund or implied, with respect to the accu- racy, completenem, or watalanss of the information contained in a report, or whes the wo of my infor. attoa, apparatus, method, or procesi di cloud la o report may not latring printoly owned rights; or B. Askumo wy liabilidas mu roopect to the une of, or for dumme ronde Iron Whe Nm of uy taformation appartawe, method, or proces daclound in Wo report. Al wand in the shore, "perna acting a behalf of the communion" locirdes av en. plogne or contractor of the Coundation, or employee of much contractor, to the extent that nch onployna or contractor of the Commission, or rapion of much contractor preparos, denminstes, or provides accou to, any buformation pursuant to do employment or contract with the Commission, or as employmeat with such contractor. probe. The radiation also indicates variation of density by the stepwise increases at regions containing many turns and high energy gain. 'The choice of the combination of electro- static and electromagnetic elements was made because of the desire to avoid the necessity for very high potential gradients and the resulting operational problems. The magnetic eler..ents provide at these onergies much stronger ex- ' traction forcos than could possibly be obtained with an eloctrostatic system. The lower gradient requirement of the electrostatic deflector also allows the use of the graphite septum which makes the radiation problems inside the cyclotron far more tractable. It is possible to go into the cyclotron and perform work for several hours within a short time (8 hours) after the cyclotron has been operated with 30 microamperes of 60 MeV protons for sevoral days. Energy Spread The energy spread of the beam from ORIC as been measured by two in othods. Both systems utilize the 6-ft radius 153° analy ,ing ma gaet and the entrance-slit systern. The first method scat- ters beam at 15° from a 1/2 mm wide plastic target at the image slit position of the analyzing magnet. The beam is prepa:ed by the object slit to present a lamm wide beam to the magnet. A detector, carefully collimated, sees only the scattered beam. The primary beam is moved by the magnet so that first one edge of the analyzed team is striking the target and then the other edge of the beam strikeo the target. The peak separation of the two scattered beams is observed on a pulse height analyzer. Use of a plastic tar- get containing carbon allows the 4.43 MOV ex- cited state to be used as a calibration. It wao found that this measurement was consistent with a method which simply uses the energy difference corresponding to a magnet current difference obtained by adjusting the two edges of the analyzed team to the center of a phosphor screen. Alpha particles at full energy, 80 MeV, have an energy spread of 3.3% fuil width at half n.aximum. Protons are not so well defined, with an energy spread of 1% FWHM for 60 MeV. The probable reasons for this are discussed later. For values of v. greater than 1.0 the cor- responding values of , are small, hence the axial width of the beam is larger and the current density on the septum is smaller than they would be after v = 1. This reduces the problems associated with maintaining and cooling the sep- tum. Axially, the beam is extremely well defined as it leaves the cyclotron, with a virtual source that appears to be located near the ion source. Phase-Width Measurements External Beam Characteristics Emittance Measurements Both internal and external phase width of the beanı have been measured. Results are very consistent for e/m = 1/2 particles, but some diffi. culty was encountered in resolving the différences in the proton cases. The dovelopment of a high- resolution system for measurement of the mxternal pulse width has allowed an explanation for the differences in these measurements. Internal Phase Width The radial and axial emittance, as calcula. ted from measurements made on an exiracted beam of 36.6 MeV H Hare 55 millimeter-milli- radian, in the radial direction and only 14 millimeter-milliradians in the axial direction. The radial virtual source is in the middle of the compensated -iron channel and has an apparent size of 2.5 mm. The interesting possibility of compensating for the remaining field gradient in the region where the beam leaves the iron channel and eliminating the need for Quadropole Number I by having an essentially parallel beam leaving the cyclotron has been considered. The internal phase width of the beam has been measured using the method described by Smith and Garren. Results of these measure. ments are shown in Figures 3 and 4 for 40 MeV protons and 68-MeV alpha particles. The phase width for both corresponds to A sine ø = 0.3, or about 20° for isochronous conditions. The phase history shown is not particularly good but ex- traction officiency for both beams is over 50%. if several turns are extracted each would have a different phase and would be spread in time in the external beam. External Phase Width Conclusions The extraction system for ORIC has proved to be stable, reliable, and easy to operate. The harmonic necessary to achieve the coherent oscillations is not easy to calculate but can be obtained in operation and optimized without difficulty. To measure the width of the external beam pulse a 50-ohm constant-impedance Faraday cup was designed and constructed. This device is water-cooled and capable of collecting heam currents in excess of 25 microampares. The signal generated in this device is delivered to a sampling oscilloscope with a 50 ohm input termi- nation and a rise time of 0, 35 nanoseconds. Typical pulse widths observed for 22, 40 and 60- MEV protons and 47, 68 and 80-MeV alpha particles are shown in Figure 5, with halt maxi- mum pulse widths of 36°, 33°, 30°, 180, 230, and 20°,rospectively. The time scale is 2 nano- seconds pe: division for all but the 47 MeV alpha particles, which is 5 nanoseconds per division. Alpha particles have nearly ideal conditions for extraction and tiie extraction efficiencies and beam quality obtained indicate that this extreulion method can be very successful. Comparison of Internal and External Measure- ments The larger vr, typically 1.08, for higher energy protons and the smaller change in radius per turn compress the coherent oscillations so that it is more difficult to extract the beam with high efficiericy. Reduction of the phase width of the beam by the incorporation of defining slits allows the possibility of improving sxtraction conditions for protons by increasing the co- herence of the beam. References 1. R. S. Lord, et. al., CERN 63-19, 297 (1963). 2. A. A. Garren and Lloyd Smith, CERN 63-19, 18 (1963;. Within the limits of the resolution of the measurements, the results of the internal and external measurements for the alpha beam case aro quite con sistent. The protons at 46 MOV, with a nearly identical internal phase width, show a much wider external phase widt, than seems reasonable. It was decidec to investigate the external pulse width of the 40-MeV protons in greater detail since this particular beam, partial- ly because of better extraction efficiency and higher beam current available, displayed come structure in the external beam pulse. Some of the resuits of these tests are shown in Figure 6. The horizontal time scale for these pictures is 2. nanoseconds per division, and each represents the shape of the external beam pulse for the dee voltage as given. The dee voltage is adjusted for maxinim extracted beam for each case and the extracted beam goes to zero between each value. This indicates that each picture repre- sents extraction on a different precessional cycle and represents a condition uf dee voltage for minimum seam density at the septum entrance. The pulse structure can be explained by referring to Figure 3 and noting that a rapid phase ex- cursion occurs at the extraction radius so that H46-4701 . ----Turbim! .. .is: : :::::: : ELECTROSTATIC CHANNEL- CD-10 COAXIAL MINONETIC CHANNEL -------- C COMPENSATED IRON CHANNEL -VERTICAL MOBITIONING MAGMET QUADRUSOLE Ne, 6 VERTICAL MOBI: 10WINO MAOMET 2 . s core... - - .. ... . .. . . . --- - - - - Fig. 1. The Beam Extraction System in ORIC. Tºp lo t ilie i: PMTT : -. + , ..!.!...f . - : ..: . -.. totis todoins ATING BHANTI- Titt.tthon #2 S Fl:titit: -- TERET 2017 . .....: Lota 14.1: Hibertsitatea i tit. 1 to go .. l. fi... c DEFLEGTED BEAM TITONTT2T 11 169% OF CIRC. BEAM - - litatori :: Loan BEAM 11 EFERE-+-+ - isi sir po DIRECTION -- - TCULATING I ONILIH BEAM CURRENT (rel) : .. L ... ... no-com :-:-: .. . ... . .co Min. . 2 m.- IM "Kahit ..torrent RADIATION 11. DEFLECTED BEAM TEL. CURRENT ONI: vieta NORTH PROBE . . - - t...*. BEAM CURRENT . :: . . . .. . . .. - - ] - i C i i 22, 23 i 24 25 26 27 : "28 29 30 in... 01..imibai .. ..o NORTH PROBE RADIUS (in.] Fic 2 Beam Current on the Movable Probe as a Function of Radius. Radiation was measured with a neutron detector located in the cyclotron vault. . - 3-2-0801310 ошо - no Bor ........... ........ 22 MeV 3 MOV 40 M.V PROTONS H ... o . °•........ 3 . ...ooo .. - .. 47 NOV NO MOV ALPHA PARTICLES NAOIVI finis) Fig. 5. - - Fig. 3. Phase va. Radius Measured for 40 MeV Protons. External Beam Pulse Width: ior Protons and Alpha Particles of. Various Energier. All horizontal 8.ales are 2 nsec par division except 47 MeV alpha particles which is 5 nsec per division. - - 100 - - - - - - - - - E--- SALV aina LATED MEMOV ALMA . • . ... ..o 000.00 ooo O00000 oso TIV . 80 RADIUS (inches) Fig. 4. Phase vs Radius Measured for 68 MeV Alpha Particles. Fig. 6. External Beam Pulse Shapes for 40 MeV Protons. Dee voltage is varied, each picture represents extraction on a different precessional cycle. - i -- > " ie. * CS . 1.1 NY 1 ) ILI WWW N - Ly. 1 F WANI > .. .. . . . END DATE FILMED 7/27 / 66 T ..... " i -